1. Field of the Invention
The present invention relates to the performance of monolithic microwave integrated circuit ("MMIC") amplifiers and more particularly to a biasing circuit for a field effect transistor ("FET").
2. Discussion of the Prior Art
In a typical two stage MMIC amplifier, two FETs sharing the same biasing source are connected in series at microwave frequencies. In order to impede the flow of microwave energy and the resultant losses thereof while still allowing DC current to flow, an element must be placed between the stages. There are two well known solutions, use of an inductor or a resistor.
Use of an inductor limits the bandwidth that is achievable because at low frequencies, the inductor must be large enough to stop the signal, whereas at high frequencies, the inductor must be small to stop in band resonances. Thus, the choice of bias circuitry influences the bandwidth that is achievable. Further, inductors are physically large and their performance is improved with increased size. This is clearly undesirable in MMIC fabrication where the goal is to make the circuit as small as possible.
A resistor is a smaller device and bandwidth considerations are absent. However, there are bias constraints. If the resistor is large enough so that the signal is not loaded down, then relatively large voltages are required. A number of systems could not accommodate this device as most systems have +12 V or less available. Further, significant power is wasted, making the chip inefficient. If the resistor chosen is small, then less voltage is required, but a low impedance path to ground is presented. Consequently, both gain and efficiency suffer. Another problem with using a resistor is that in mass fabrication of circuits, the pinchoff voltage of the FET cannot be held to close tolerances, thus the transistor will bias at different operating points. Consequently, both the amount of current going to the first device and the percentage of IDSS current the device sees are variable and dependent on difficult to control fabrication factors. This variation in current translates to a variation in DC voltages across the biasing resistor. The total voltage must be shared between the two FETs and the biasing resistor. If the resistor takes too much voltage, the FETs will lose G.sub.m and, in extreme cases, the FETs may not operate at all. Where the left over voltage is marginal, the device will be subject to temperature failure.